Noise reduction apparatus

ABSTRACT

Effective reduction is made of direct sound or diffracted sound of a noise propagating from plural directions. Three microphones ( 18   k−1   , 18   k   , 18   k+1 ) are disposed and: noise is detected by each of the microphones; delay signals are generated by delaying plural times a signal that has been output from 2 of the microphones ( 18   k−1   , 18   k+1 ); the output signal from the remaining microphone ( 18   k ) is added to 2 of the delayed signals and input to respective speakers of a speaker unit; a control sound is output from the speaker unit in 3 directions (L, C, R); and reduction is made of direct sound or diffracted sound of a noise propagating from each of the directions.

TECHNICAL FIELD

The present invention relates to a noise reducing device, and inparticular to a noise reducing device for reducing, at the outside of asound barrier, noise that is generated when moving bodies move, forexample, noise that has been generated by vehicles traveling on anexpressway.

BACKGROUND ART

Conventionally known in which speakers are disposed at the top edge ofthe sound insulation barrier and along the top edge of the soundinsulation barrier, and noise that has been diffracted from the soundinsulation barrier is suppressed by sound waves of opposite phase beingemitted from the speakers.

Also, a noise reducing device is known in which, when suppressing noiseusing a control sound, linear sound source arrays of speaker unitsincluding plural arrayed speakers are disposed along the top edge of asound barrier, and by using filter coefficients that have been computedby explicit methods and an acoustic signal of the noise that has beendetected by a microphone, noise is reduced at a control point byemitting a control sound of the opposite phase to diffracted sound ofthe noise.

Furthermore, an ASE system manufactured by Mitsubishi Heavy Industriesis known as a practical method of active control using a linear arraysound source for reduction of sound barrier diffracted sound from asound source, sound passing through the interior of a continuouslydisposed sound barrier. The ASE system is one of localized control thatassumes that the control points are points at the apex of a soundbarrier, and since the ASE system units function independently, thecontrol area may be extended by disposing a series of ASE system units,and the problem of mutual interference between control sounds does notoccur.

However, in the above described technology, since linear array soundsources such as speakers with speakers arranged in a straight line areused, then the control sound is emitted from each of the speakerssimultaneously, sound emission only has a strong sound pressure level ina single direction. Due to this, when there are moving noise sources,such as the noise from vehicles traveling on an expressway, since noisepropagates to the noise reduction control points from variousdirections, it is difficult to carry out effective noise reduction.

Patent Publication 1: Japanese Patent Application No. H9-54593

Patent Publication 2: WO03/030147 A1

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The problem to be solved by the invention is that effective noisereduction of noise propagating from various directions cannot be made.

Method of Solving the Problem

The first invention is a noise reducing device including a speaker unit,a first microphone, a second microphone, first control signal outputmeans, second control signal output means and control means. The speakerunit includes plural speakers arrayed so that the direction of soundemission therefrom faces in a predetermined direction. The firstmicrophone is disposed so as to correspond to the speaker unit, andcollects the sound of a noise and outputs a first acoustic signalcorresponding to the sound of the noise that has been collected. Thesecond microphone is disposed in a position that is separated in thespeaker array direction from the disposed position of the firstmicrophone, and it collects the sound of a noise and outputs a secondacoustic signal corresponding to the sound of the noise that has beencollected. The first control signal output means outputs, on the basisof the first acoustic signal, a first control signal for emitting acontrol sound that reduces the direct sound or the diffracted sound ofthe noise that has been collected by the first microphone. The secondcontrol signal output means outputs, on the basis of the second acousticsignal, plural second control signals for emitting a control sound thatreduces the direct sound or the diffracted sound of the noise that hasbeen collected by the second microphone, the plural second controlsignals having each been delayed respectively by incrementalpredetermined time periods. The control means controls such that thefirst control signal is input simultaneously to the respective speakers,and such that the second control signals are input with graduallyincreasing delay times from the speaker at one end of the speaker unittoward the speaker at the other end of the speaker unit.

The second invention is a noise reducing device including a speakerunit, a first microphone, a second microphone, a third microphone, firstcontrol signal output means, second control signal output means, thirdcontrol signal output means and control means. The speaker unit includesplural speakers arrayed so that the direction of sound emissiontherefrom faces in a predetermined direction. The first microphone isdisposed so as to correspond to the speaker unit, and collects the soundof a noise and outputs a first acoustic signal corresponding to thesound of the noise that has been collected. The second microphone isdisposed in a position that is separated in the speaker array directionfrom the disposed position of the first microphone, and collects thesound of a noise and outputs a second acoustic signal corresponding tothe sound of the noise that has been collected. The third microphone isdisposed in a position that is separated in the speaker array directionfrom the disposed position of the first microphone in the oppositedirection to that of the second microphone, and collects the sound of anoise and outputs a third acoustic signal corresponding to the sound ofthe noise that has been collected. The first control signal output meansoutputs, on the basis of the first acoustic signal, a first controlsignal for emitting a control sound that reduces the direct sound or thediffracted sound of the noise that has been collected by the firstmicrophone. The second control signal output means outputs, on the basisof the second acoustic signal, plural second control signals foremitting a control sound that reduces the direct sound or the diffractedsound of the noise that has been collected by the second microphone, theplurality of second control signals having each been delayedrespectively by incremental predetermined time periods. The thirdcontrol signal output means outputs, on the basis of the second acousticsignal, plural third control signals for emitting a control sound thatreduces the direct sound or the diffracted sound of the noise that hasbeen collected by the third microphone, the plurality of third controlsignals having each been delayed respectively by incrementalpredetermined time periods. The control means controls such that thefirst control signal is input simultaneously to the respective speakers,such that the second control signals are input with gradually increasingdelay times from the speaker at one end of the speaker unit toward thespeaker at the other end of the speaker unit, and such that the thirdcontrol signals are input with gradually decreasing delay times from thespeaker at the one end of the speaker unit toward the speaker at theother end of the speaker unit.

The third invention is a noise reducing device including plural speakerunit, plural microphones, first control signal output means, secondcontrol signal output means, third control signal output means andcontrol means. The plural speaker units each include plural speakersarrayed so that the direction of sound emission therefrom faces in apredetermined direction. The plural microphones are disposed so as tocorrespond to the respective speaker units, and collecting the sound ofa noise and outputting an acoustic signal corresponding to the sound ofthe noise that has been collected. The first control signal output meansare provided to correspond with each of the respective speaker units,and output, on the basis of the acoustic signal that has been outputfrom a first corresponding microphone that corresponds to thecorresponding speaker unit, a first control signal for emitting acontrol sound that reduces the direct sound or the diffracted sound ofthe noise that has been collected by the first corresponding microphone.The second control signal output means are provided to correspond witheach of the respective speaker units, and output, on the basis of theacoustic signal that has been output from a second correspondingmicrophone that corresponds to one of the speaker units that is adjacentto the corresponding speaker unit, a second control signal for emittinga control sound that reduces the direct sound or the diffracted sound ofthe noise that has been collected by the second correspondingmicrophone. The third control signal output means are provided tocorrespond with each of the respective speaker units, and output, on thebasis of the acoustic signal that has been output from a thirdcorresponding microphone that corresponds to the other one of thespeaker units that is adjacent to the corresponding speaker unit, athird control signal for emitting a control sound that reduces thedirect sound or the diffracted sound of the noise that has beencollected by the third corresponding microphone. The control means isprovided so as to input the signals to the respective speaker units, andcontrols such that the first control signal is input simultaneously tothe respective speakers of the speaker units to which the signals arebeing input, such that the respective second control signals are inputto the speakers of the speaker unit to which the signals are being inputwith gradually increasing delay times from the speaker at one end of thespeaker unit to which the signals are being input toward the speaker atthe other end thereof, and such that the third respective controlsignals are input to the speakers of the speaker unit to which thesignals are being input with gradually decreasing delay times from thespeaker at the one end of the speaker unit to which the signals arebeing input toward the speaker at the other end thereof.

That is to say, the main feature of the present invention is having atleast two types of control sound, control sound that is emittedsimultaneously from each of the speakers of a speaker unit, and controlsound that is emitted by each of the speakers delayed by increments of apredetermined amount, or in other words plural control sounds that havedifferent propagation directions from each other.

EFFECT OF THE INVENTION

In the noise reducing device of the present invention, since controlsound is emitted in plural directions from speaker unit(s), effectivenoise reduction may be made even for noise propagated from pluraldirections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral view of a noise reducing device in a state of beingmounted to a sound barrier (first exemplary embodiment).

FIG. 2 is a lateral view of a noise reducing device in a state of beingmounted to a sound barrier (second exemplary embodiment).

FIG. 3 is an explanatory diagram showing a speaker unit into which isinput a control signal that has been generated from a signal output fromone microphone (first exemplary embodiment).

FIG. 4 is a circuit diagram showing details of a control circuit (firstexemplary embodiment).

FIG. 5 is an explanatory diagram showing paths of diffracted sound ofnoise propagating to control points.

FIG. 6 is an explanatory diagram showing the progression of soundpressure level with time of noise at control points.

FIG. 7 is a block diagram showing a filter portion of a noise reducingdevice (second exemplary embodiment).

FIG. 8 is a block diagram showing a filter portion of a noise reducingdevice (third exemplary embodiment).

FIG. 9 (1) to (5) show example modifications showing speaker unit andmicrophone placement positions.

BEST MODE OF IMPLEMENTING THE INVENTION

Explanation will be given below of details of exemplary embodiments ofthe present invention with reference to the drawings.

First Exemplary Embodiment

The present exemplary embodiment is where the present invention isapplied to reducing diffracted noise cut from an expressway soundbarrier and propagated to the outside of the sound barrier, by using acontrolled sound source (secondary sound source) placed to the outsideof the sound barrier.

As shown in FIG. 1, there is a sound insulation plate 12 attachedhorizontally and continuously at the top edge of an expressway soundbarrier 10 and along the top edge of the sound barrier 10. The soundinsulation plate 12 contributes to preventing the generation of howlingbetween the sensor/microphone and the controlled sound source (secondarysound source). The sound insulation plate 12 is not always necessary,and a howling prevention circuit may be introduced for controlling thegeneration of howling. Plural speaker units 14 ₁, 14 ₂, 14 ₃, . . . 14_(k−1), 14 _(k), 14 _(k+1), . . . , 14 _(n) (k and n are positiveintegers with k<n) are mounted at the vicinity of an edge of the soundinsulation plate 12 and at the outside of the sound barrier on the topface of the sound insulation plate 12, and each of the speaker units isplaced adjacently in a series. Each of the speaker units may be disposedwith a predetermined interval therebetween, or may be disposed so as tobe touching each other.

As shown in FIG. 3, each of the speaker units is configured from pluralspeakers 16 ₁, 16 ₂, 16 ₃, . . . 16 _(m) that are arrayed in a straightline, arrayed so that the sound emission therefrom all face in the samedirection. The sound emission direction of a single control soundemitted out from respective speakers of the present exemplary embodiment(the arrow direction allocated C, described later) is the propagationdirection of the diffracted sound from the noise that has been emittedonto the front face of the sound barrier and is being diffracted, thatis to say, the direction orthogonal to the array direction of thespeakers, and inclined in a downward direction at an angle (see FIG. 1).

In the present exemplary embodiment, since the speaker units are arrayedin a straight line, the above sound emission directions of the speakerunits are all the same direction. Meanwhile, the above sound emissiondirections of the speaker units are different for each of the speakerunits if the sound barrier is curved and the speaker units are arrangedalong the sound barrier.

At the vicinity of an edge of the sound insulation plate 12 that is tothe inside of the sound barrier and at the bottom face of the soundinsulation plate 12, as shown in FIG. 1 and FIG. 2, sound of propagatednoise generated at the inside of the sound barrier is collected, andthere are plural microphones 18 ₁, 18 ₂, 18 ₃, . . . 18 _(k−1), 18 _(k),18 _(k+1), . . . , 18 _(n), serving as sensors/microphones outputting anacoustic signal corresponding to the collected sound of the noise,provided so as to correspond to the respective speaker units. It ispreferable that the positions corresponding to each of the microphonesare at central portions of the speaker units. It is preferable to useultra-directional microphones as such microphones.

Furthermore, there are plural control circuits 20 ₁, 20 ₂, 20 ₃, . . .20 _(k−1), 20 _(k), 20 _(k+1), . . . , 20 _(n) that output controlsignals for emitting from the speaker units control sound, for reducingthe sound collected at the microphone that is diffracted by the soundbarrier, the control circuits 20 ₁, 20 ₂, 20 ₃, . . . 20 _(k−1), 20_(k), 20 _(k+1), . . . , 20 _(n) being disposed so as to correspondrespectively with the plural speaker units. Such control circuits may beconfigured with a DSP. Respective microphones are connected atrespective input terminals of the control circuits, and each respectivemicrophone of the speaker units is connected at respective outputterminals of the control circuits. Also, respective control circuits areconnected to so as to input and output signals as shown in FIG. 4.

In FIG. 1 the k^(th) microphone, speaker unit, and control circuit areshown.

Next, details will be explained of the respective control circuits, withreference to FIG. 4. Since the respective control circuits are of thesame configuration, explanation will be given of the k^(th) controlcircuit 20 _(k) and explanation of the configuration of the othercontrol circuits will be omitted. In FIG. 4 the configurations of thecontrol circuits adjacent to the control circuit 20 _(k) are drawn withsome parts omitted, but the configuration thereof is the same as that ofcontrol circuit 20 _(k). As is shown in the figure, in the controlcircuit 20 _(k) there are provided 3 analog-digital convertors (A/Dconvertors) 30 _(Rk), 30 _(Ck) and 30 _(Lk) that convert analog signalsinto digital signals. The microphone 18 _(k) is connected to the inputterminal of the analog-digital convertor 30 _(Ck).

Inverse filters 32 _(R), 32 _(C) and 32 _(L) are connected to the outputterminal of the analog-digital convertor 30 _(Ck), and the inversefilters 32 _(R), 32 _(C) and 32 _(L) each carry out digital filteringusing the digital signal input from the analog-digital convertor 30_(Ck) together with one or other of the filter coefficients W_(R), W_(C)and W_(L) that have been determined in advance, and the inverse filters32 _(R), 32 _(C) and 32 _(L) respectively output control signals ofreverse phase relative to the digital signal input from theanalog-digital convertor 30 _(Ck). In the present exemplary embodiment,the respective values of the filter coefficients W_(R), W_(C) and W_(L)are set to be equivalent to each other, but the values may be set so asto be different from each other.

The filter coefficients W_(R), W_(C) and W_(L) may be computed bymeasuring the noise in advance, and be filter coefficients forgenerating a control signal for emitting control sound of the oppositephase relative to the measured noise. Or, the microphone may be disposedat the control point, and in the state in which diffracted sound fromthe noise is propagated to the control point, the control sounds may beemitted from the respective speakers and the filter coefficients set, asthe filter coefficients are adjusted of the inverse filters, as theinverse filters when the output from the microphone disposed at thecontrol point is minimized. In both of the methods for setting thefilter coefficients, control signals are generated for emitting controlsounds for reducing the diffracted sound of the noise.

The output terminal of the inverse filter 32 _(C) is connected toaccumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) that are provided in thesame number m to that of the speakers of the speaker unit and each havethree inputs and one output. The respective accumulators add togetherthe 3 input signals and output the result.

The output terminals of the accumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m)are connected to the respective speakers 16 ₁, 16 ₂, 16 ₃, . . . 16 _(m)of the speaker unit 14 _(k) through digital-analog convertors (D/Aconvertors) 36 ₁, 36 ₂, 36 ₃, . . . 36 _(m) for converting digitalsignals into analog signals, and through amplifiers 38 ₁, 38 ₂, 38 ₃, .. . 38 _(m) for increasing the amplitude of the input signal.

The output terminal of the inverse filter 32 _(L) is connected throughan digital-analog convertor 46 _(k) to the speaker unit 14 _(k−1) thatis one of the adjacent speaker units to speaker unit 14 _(k) so as toemit control sound in the L direction therefrom, and is connectedthrough an analog-digital convertor (A/D convertor) 30 _(L(k−1)) of acontrol circuit 20 _(k−1) corresponding to the speaker unit 14 _(k−1),and through delay circuits for emitting control sound in the L directiondelayed by necessary delaying times Δ₁, Δ₂, Δ₃, . . . Δ_(m), to theaccumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) of the control circuit 20_(k−1).

The output terminal of the inverse filter 32 _(R) is connected throughdigital-analog convertor 46 _(k) to the speaker unit 14 _(k+1) that isthe other of the adjacent speaker units to speaker unit 14 _(k) so as toemit control sound in the R direction therefrom, and is connectedthrough an analog-digital convertor (A/D convertor) 30 _(R(k+1)) of acontrol circuit 20 _(k+1) corresponding to the speaker unit 14 _(k+1),and through delay circuits for emitting control sound in the R directiondelayed by necessary delay times Δ₁, Δ₂, Δ₃, . . . Δ_(m), to theaccumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) of the control circuit 20_(k+1).

The respective control circuits of the digital-analog convertor 44 _(k),46 _(k), and the analog-digital convertors (A/D convertors) 30 _(Lk), 30_(Rk) are not necessary if the operation of the digital signalprocessing of the serially disposed control circuits, of control circuit20 _(k−1), control circuit 20 _(k), control circuit 20 _(k+1) etc., areoperated according to the same master clock. In such a case each of thecontrol circuits are connected directly, and not through adigital-analog convertor and an analog-digital convertor.

Therefore, FIG. 4 is an example in which all of the serially disposedcontrol circuits control circuit 20 _(k−1), control circuit 20 _(k),control circuit 20 _(k+1) etc. have their own respective independentmaster clocks.

The output terminal of the analog-digital convertor 30 _(Rk) isconnected to respective delay elements 40 ₁, 40 ₂, 40 ₃, . . . 40 _(m)that are provided to a delay circuit in the same number m to the numberof the speakers of the speaker unit and that delay input singles withrespective incremental delay times of Δ₁, Δ₂, Δ₃, . . . Δ_(m). The delaytimes of Δ₁, Δ₂, Δ₃, . . . Δ_(m) may be, for example, set to 0, τ, 2τ, .. . (m−1)τ.

The output terminal of the analog-digital convertor 30 _(Lk) isconnected to respective delay elements 42 ₁, 42 ₂, 42 ₃, . . . 42 _(m)that are provided to a delay circuit in the same number m to the numberof the speakers of the speaker units and that delay input singles withrespective incremental delay times of Δ₁, Δ₂, Δ₃, . . . Δ_(m). The delaytimes of Δ₁, Δ₂, Δ₃, . . . Δ_(m) may be, for example, set to 0, τ, 2τ, .. . (m−1)τ.

Respective delay elements 40 ₁, 40 ₂, 40 ₃, . . . 40 _(m) are connectedto the input terminals of the respective accumulators 34 ₁, 34 ₂, 34 ₃,. . . 34 _(m). By doing so, delayed control signals are input, throughthe adding circuits, the digital-analog convertors, and the amplifiers,in sequence to each of the speakers of the speaker units from thespeaker at one end of the speaker units toward the speaker at the otherend thereof, from the signal with the smallest delay time to the signalwith the largest delay time, with the delay times gradually gettinglonger.

Also, respective delay elements 42 ₁, 42 ₂, 42 ₃, . . . 42 _(m) areconnected to inputs of the accumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m)in the opposite manner to the connection of the delay elements 40 ₁, 40₂, 40 ₃, . . . 40 _(m) to the accumulators. That is to say, therespective delay elements 42 ₁, 42 ₂, 42 ₃, . . . 42 _(m) are connectedsuch that the accumulator that has been input with the delayed controlsignal output with the smallest delay time output from the delayelements 40 ₁, 40 ₂, 40 ₃, . . . 40 _(m), is input with the controlsignal with the largest delay time, and the accumulator that has beeninput with the delayed control signal output with the largest delay timeoutput from the delay elements 40 ₁, 40 ₂, 40 ₃, . . . 40 _(m), is inputwith the control signal with the smallest delay time.

By doing so, delayed control signals are input, through the addingcircuits, the digital-analog convertors, and the amplifiers, in sequenceto each of the speakers of the speaker units from the speaker at the oneend of the speaker units toward the speaker at the other end thereof,from the signal with the largest delay time to the signal with thesmallest delay time with the delay times gradually getting shorter.

As a result of configuring each of the control circuits as above, asshown in FIG. 3, an acoustic signal that has been output from onemicrophone 18 _(k) is input to the speaker unit 14 _(k) through thecontrol circuit 20 _(k), and also input to speaker unit 14 _(k−1)through control circuit 20 _(k) and control circuit 20 _(k−1),corresponding to speaker unit 14 _(k−1) that is one of the speaker unitsadjacent to speaker unit 14 _(k), and input to speaker unit 14 _(k+1)through control circuit 20 _(k) and control circuit 20 _(k+1),corresponding to speaker unit 14 _(k+1) that is the other of the speakerunits adjacent to speaker unit 14 _(k).

Next, explanation will be given of the operation of the presentexemplary embodiment. As shown in FIG. 5, if the control point forreducing the noise is P, then a portion of the noise that is emitted dueto vehicles travelling on the expressway is blocked by the soundbarrier, but other portions of the noise are diffracted by the top edgeside of the sound barrier, and the diffracted sound is propagated to thecontrol point P from directions of straight ahead and from both the leftand right directions.

Noise that is generated by vehicles travelling on the expressway iscollected at respective microphones, and acoustic signals correspondingto the noise that has been collected from respective microphones isoutput, converted into digital signals by the respective analog-digitalconvertors that are connected to the microphones, and input to therespective inverse filters. In the respective inverse filters, digitalfiltering is carried out using the digital signal that has been inputand the filter coefficients that have been set in advance, and a controlsignal is generated and output that is of the reverse phase relative tothe digital signal that has been input from the analog-digitalconvertor.

Next, explanation will be given of the operation of each of the controlcircuits in FIG. 4 using the control circuit 20 _(k) as representativethereof. In the control circuit 20 _(k) the control signal that has beenoutput from the inverse filter 32 _(C) is simultaneously input to therespective accumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m), converted intoanalog signals by the respective digital-analog convertors 36 ₁, 36 ₂,36 ₃, . . . 36 _(m), and simultaneously input to the respective speakersof speaker unit 14 _(k) via the respective amplifiers 38 ₁, 38 ₂, 38 ₃,. . . 38 _(m). By doing so, control sound is emitted from the speakerunit in a direction that is orthogonal to the direction in which thespeakers are arrayed in the speaker units and in a direction that isinclined downward (in the arrow direction designated with C) and thediffracted sound of the noise propagated in the arrow directiondesignated C is reduced at the control point. Here, the plane of thecombined waves of the control sound is parallel to the array directionof the speakers in the speaker unit.

Furthermore, the control signal that has been output from the inversefilter 32 _(R) of the control circuit 20 _(k−1) is input to the delaycircuit through the digital-analog convertor 44 _(k−1) and theanalog-digital convertor 30 _(Rk) of the control circuit 20 _(k), it isdelayed by the respective incremental delay times of Δ₁, Δ₂, Δ₃, . . .A_(m), and input to the respective accumulators 34 ₁, 34 ₂, 34 ₃, . . .34 _(m). Here, the respective control signals that have been delayed(delayed control signals) are input into the respective accumulators 34₁, 34 ₂, 34 ₃, . . . 34 _(m) in the sequence from the shortest delaytime. The delayed control signals that have been output from theaccumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) are converted into analogsignals by the respective digital-analog convertors 36 ₁, 36 ₂, 36 ₃, .. . 36 _(m), and input into the respective speakers of the speaker unit14 _(k) via the respective amplifiers 38 ₁, 38 ₂, 38 ₃, . . . 38 _(m).By doing so, each of the speakers of the speaker unit are input withrespective delayed control signals so that the delay time gradually getslonger from the speaker at the one end of the speaker unit toward thespeaker at the other end. By doing so the control sound that is emittedfrom each of the speakers is combined, and the plane of the combinedwave is formed in a direction that is inclined at an angle θ, shown inthe formula below, relative to the arrayed direction of the speakers ofthe speaker units, and the control sound is emitted in the arrowdirection designated with R. Thus, since the control sound emitted whichhas been inclined at angle θ becomes equivalent to that from thehypothetical sound source A (see FIG. 3), the diffracted sound of thenoise propagating in the arrow direction designated by R is reduced atthe control point.

[Equation 1]

θ=sin⁻¹{(Δ_(m)−Δ₁)c/D}  (1)

Wherein in the above, c is the speed of sound of the control sound, D isthe length of the speaker unit in the speaker array direction, and θ ispositive in the clockwise direction with reference to the speaker arraydirection.

Furthermore the signal output from the inverse filter 32 _(L) of thecontrol circuit 20 _(k−1) is input into the delay circuit through thedigital-analog convertor 46 _(k) and the analog-digital convertor 30_(Ck) of the control circuit 20 _(k), delayed by respective incrementaldelay times of Δ₁, Δ₂, Δ₃, . . . Δ_(m), and input to the respectiveaccumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m). Here, the respectivedelayed control signals are input into the respective accumulators 34 ₁,34 ₂, 34 ₃, . . . 34 _(m) in the sequence from the longest delay time,the reverse order to the above. The delayed control signals that havebeen output from the accumulators 34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) areconverted into analog signals by the respective digital-analogconvertors 36 ₁, 36 ₂, 36 ₃, . . . 36 _(m), and input into therespective speakers of the speaker unit 14 _(k) through the respectiveamplifiers 38 ₁, 38 ₂, 38 ₃, . . . 38 _(m). By so doing, delayed controlsignals are input into each of the speakers of the speaker units withgradually shortening delay times from the speaker at the one end to thespeaker at the other end. Thereby, the sound emitted from each of thespeakers is combined into a control sound, with a combined wave frontformed at an inclination angle of (π−θ) to the array direction of thespeakers in the speaker units, and the control sound is emitted in thedirection of the arrow allocated letter L. Therefore, the diffractedsound of the noise propagating in the direction of the arrow allocatedthe letter L is reduced at the control point since the control soundemitted at an inclination angle of (π−θ) is equivalent to that from thehypothetical sound source B (see FIG. 3).

If the distance from the microphone 18 _(k) to the speaker unit 14 _(k)is d1, and the distance to the hypothetical sound source A formed by thespeaker unit 14 _(k+1) and to the hypothetical sound source B formed bythe speaker unit 14 _(k−1) is d2, and the respective processing time ofthe control circuits are T (processing time is set to be approximatelyequivalent to an excess of the time to pass through the analog-digitalconvertor and digital-analog convertor, for forming the hypotheticalsound source), and, since d1<d2, it is preferable that the controlcircuit processing time is determined so as to satisfy the followingequation.

[Equation]

d1/c>T  (2)

Since control sound is emitted from each of the respective speakerunits, as explained above, in the direction of the arrow allocatedletter R, the direction of the arrow allocated letter C, and thedirection of the arrow allocated letter L, the diffracted sound isreduced of the noise that is propagating to the front, and to the leftand the right of the control point.

Explanation will next be given regarding the lengths of the controlsections that are able to reduce the noise in the present exemplaryembodiment. As shown in FIG. 5, noise that is generated from the point Ato the point D by vehicles travelling in the direction of the arrow onthe expressway, and that moves with the vehicles as they travel, isdiffracted by the sound barrier and propagates to the control point Pfrom the respective directions of the front, left and right. Therefore,the noise level is highest from the point C that is the closest to thecontrol point, and the noise level is lowest from the points A and Dthat are the furthest from the control point. When considering thereduction of a noise level that is at a prescribed sound pressure level(for example, 70 dB) or above, that is to say the total noise levelgenerated from the point A to point D, for the time that the noise levelcontinues at the prescribed sound pressure level or above to beT(seconds), in the example of the change in the noise sound pressurelevel at the control point with time in FIG. 6, if the velocity of thevehicle is V [km/h], then the length L [m] of the control section thatis able to reduce the noise at the control point P is as set out below.

[Equation 3]

L=V×T/(1000×60×60)  (3)

Second Exemplary Embodiment

Explanation will next be given of a second exemplary embodiment of thepresent invention with reference to FIG. 7. In the present exemplaryembodiment, there is an external air temperature sensor 50 disposedtherein for detecting the external air temperature, a coefficientcorrection unit 52 is provided to each of the control circuits, andcorrection is carried out in the coefficient correction unit 52 usingrespective filter coefficients according to the external airtemperature. The correction coefficients for correcting the filtercoefficients according to the external air temperature are stored inadvance in the respective coefficient correction units 52, and thecoefficient correction units 52 read out the correction coefficientsaccording to the external air temperature that has been detected andcarry out correction of the filter coefficients of the inverse filters.

By so doing, the diffracted sound of the noise may be effectivelyreduced even if the speed of sound has changed due to the external airtemperature.

An external air temperature sensor may be provided to correspond to eachof the respective control circuits, or one of the external airtemperature sensors may be disposed for each group of plural controlcircuits in single groups, or a single external air temperature sensormay be provided for a single noise reducing device.

Third Exemplary Embodiment

The third exemplary embodiment uses adaptive filters for the inversefilters 32 _(R), 32 _(C) and 32 _(L), as shown in FIG. 8, a microphone54 is arranged for detecting at the control point P the signal errorbetween the control sound and the diffracted sound of the noise, andcorrection is carried out of the filter coefficients of the adaptivefilters so that the signal error of a coefficient correction device isas small as possible.

In doing so, the diffracted sound of the noise may be effectivelyreduced even if there is a change in the environment, since control maybe carried out so that the signal error between the control sound andthe diffracted sound of the noise is as small as possible.

In the above exemplary embodiments, explanation was given of a case inwhich the diffracted sound of the noise is reduced, however, the presentinvention is applicable to reducing the direct noise that is propagatedto control points without diffraction. In such a case, control sound isemitted in the direction of propagation of the direction sound of thenoise, or in a direction that intersects with the direction of thedirect sound of the noise. Furthermore, explanation was given above of acase in which control sound is emitted in three directions, however,control sound may be emitted in two directions, and whereas explanationwas given of a case in which plural individual speaker units whereprovided, if the region for reduction of the noise is narrow, a singlespeaker unit and plural microphones may be provided.

Explanation will next be given of example modifications to the mountingpositions of the speaker units and the microphones. Explanation wasgiven above of an example in which speaker units are mounted in thevicinity of the outside edge on the top face of a sound insulation platethat is mounted horizontally to the top edge of a sound barrier, andmicrophones are mounted to the bottom face of the sound insulationplate, however, speaker units and microphones may be mounted asdescribed below. Here, explanation will be given with respect to asingle speaker unit and a microphone corresponding to this speaker unitand, since the other speaker units and microphones are similar,explanation thereof is omitted.

In the modification example in FIG. 9 (1), a sound insulation plate 12is mounted horizontally to the inside surface of the barrier at the topedge side of a sound barrier 10, and a speaker unit 14 _(k) is mountedto the top face of the sound insulation plate 12 at substantially acentral portion thereof, and a microphone 18 _(k) is disposed in aposition that is separated from the sound barrier at the lower side ofthe speaker unit and on the inside of the sound barrier. Thismodification example is favorably applied to reducing the direct soundof a noise, since the control sound is emitted inclined in an upwarddirection.

In the modification example in FIG. 9 (2), one side of a soundinsulation plate 12 is fixed to the top edge of a sound barrier 10, andthe sound insulation plate 12 is mounted horizontally to as to extendout to the inside of the sound barrier, and a speaker unit 14 _(k) ismounted to the top face of the sound insulation plate 12 at the sidethereof that is opposite to the side that is fixed to the sound barrier10, and a microphone 18 _(k) is disposed in a position that is separatedfrom the sound barrier at the lower side of the speaker unit and on theinside of the sound barrier.

In the modification example in FIG. 9 (3), one side of a soundinsulation plate 12 is fixed to the top edge of a sound barrier 10, andthe sound insulation plate 12 is mounted horizontally to as to extendout to the outside of the sound barrier, and a speaker unit 14 _(k) ismounted to the top face of the sound insulation plate 12 at the sidethat is opposite to the side that is fixed to the sound barrier 10, anda microphone 18 _(k) is disposed at the internal face of the soundbarrier at the lower side thereof.

The modification example in FIG. 9 (4) has a sound insulation plate 12fixed in a similar manner to that of the modification example of FIG. 9(3), with a speaker unit 14 _(k) is mounted to the bottom face of thesound insulation plate 12 at the side thereof that is opposite to theside that is fixed to the sound barrier 10, and a microphone 18 _(k) isdisposed in a position that is separated from the upper internal face ofthe sound barrier to the inside thereof.

The modification example in FIG. 9 (5) does not use a sound insulationplate, and mounts a speaker unit 14 _(k) along the top edge of a soundbarrier, and a microphone 18 _(k) is disposed at the lower internal faceof the sound barrier.

INDUSTRIAL APPLICABILITY

Other than the reductions that may be made to noise generated fromexpressways, as explained above, reductions may also be made of noisegenerated when moving bodies are moved such as in a railway, andfurthermore, reductions may be made in the generation of noise from astationary noise source.

EXPLANATION OF THE REFERENCE NUMERALS

-   -   10 sound barrier    -   12 sound insulation plate    -   14 ₁, 14 ₂, 14 ₃, . . . 14 _(n) speaker units    -   16 ₁, 16 ₂, 16 ₃, . . . 16 _(m) speakers    -   18 ₁, 18 ₂, 18 ₃, . . . 18 _(n) microphones    -   20 ₁, 20 ₂, 20 ₃, . . . 20 _(n) control circuits    -   32 _(R), 32 _(c), 32 _(L) inverse filters    -   34 ₁, 34 ₂, 34 ₃, . . . 34 _(m) accumulators

1. A noise reducing device comprising: a speaker unit comprising aplurality of speakers arrayed so that the direction of sound emissiontherefrom faces in a predetermined direction; a first microphone,disposed so as to correspond to the speaker unit, and collecting thesound of a noise and outputting a first acoustic signal corresponding tothe sound of the noise that has been collected; a second microphone,disposed in a position that is separated in the speaker array directionfrom the disposed position of the first microphone, and collecting thesound of a noise and outputting a second acoustic signal correspondingto the sound of the noise that has been collected; first control signaloutput means that outputs, on the basis of the first acoustic signal, afirst control signal for emitting a control sound that reduces thedirect sound or the diffracted sound of the noise that has beencollected by the first microphone; second control signal output meansthat outputs, on the basis of the second acoustic signal, a plurality ofsecond control signals for emitting a control sound that reduces thedirect sound or the diffracted sound of the noise that has beencollected by the second microphone, the plurality of second controlsignals having each been delayed respectively by incrementalpredetermined time periods; and control means, controlling such that thefirst control signal is input simultaneously to the respective speakers,and such that the second control signals are input with graduallyincreasing delay times from the speaker at one end of the speaker unittoward the speaker at the other end of the speaker unit.
 2. The noisereducing device of claim 1, wherein the control means comprises aplurality of accumulators that add together the first signal and one ofthe second control signals, and input the result of the addition to therespective speakers of the speaker unit.
 3. The noise reducing device ofclaim 1, wherein the second control signal output means comprises:output means, outputting, based on the second acoustic signal, a controlsignal that is of opposite phase to the noise that has been collected bythe second microphone; and delay means, delaying a plurality of timesthe opposite phase control signal respectively by predeterminedincremental periods of time and outputting a plurality of second controlsignals.
 4. A noise reducing device comprising: a speaker unitcomprising a plurality of speakers arrayed so that the direction ofsound emission therefrom faces in a predetermined direction; a firstmicrophone, disposed so as to correspond to the speaker unit, andcollecting the sound of a noise and outputting a first acoustic signalcorresponding to the sound of the noise that has been collected; asecond microphone, disposed in a position that is separated in thespeaker array direction from the disposed position of the firstmicrophone, and collecting the sound of a noise and outputting a secondacoustic signal corresponding to the sound of the noise that has beencollected; a third microphone, disposed in a position that is separatedin the speaker array direction from the disposed position of the firstmicrophone in the opposite direction to that of the third microphone,and collecting the sound of a noise and outputting a third acousticsignal corresponding to the sound of the noise that has been collected;first control signal output means that outputs, on the basis of thefirst acoustic signal, a first control signal for emitting a controlsound that reduces the direct sound or the diffracted sound of the noisethat has been collected by the first microphone; second control signaloutput means that outputs, on the basis of the second acoustic signal, aplurality of second control signals for emitting a control sound thatreduces the direct sound or the diffracted sound of the noise that hasbeen collected by the second microphone, the plurality of second controlsignals having each been delayed respectively by incrementalpredetermined time periods; third control signal output means thatoutputs, on the basis of the second acoustic signal, a plurality ofthird control signals for emitting a control sound that reduces thedirect sound or the diffracted sound of the noise that has beencollected by the third microphone, the plurality of third controlsignals having each been delayed respectively by incrementalpredetermined time periods; and control means, controlling such that thefirst control signal is input simultaneously to the respective speakers,such that the second control signals are input with gradually increasingdelay times from the speaker at one end of the speaker unit toward thespeaker at the other end of the speaker unit, and such that the thirdcontrol signals are input with gradually decreasing delay times from thespeaker at the one end of the speaker unit toward the speaker at theother end of the speaker unit.
 5. The noise reducing device of claim 4,wherein the second control signal output means comprises: output means,outputting, based on the second acoustic signal, a control signal thatis of opposite phase to the noise that has been collected by the secondmicrophone; and delay means, delaying a plurality of times the oppositephase control signal respectively by predetermined incremental periodsof time and outputting a plurality of second control signals; and thethird control signal output means comprises: output means, outputting,based on the third acoustic signal, a control signal that is of oppositephase to the noise that has been collected by the third microphone; anddelay means, delaying a plurality of times the opposite phase controlsignal respectively by predetermined incremental periods of time andoutputting a plurality of third control signals.
 6. A noise reducingdevice comprising: a plurality of speaker units each comprising aplurality of speakers arrayed so that the direction of sound emissiontherefrom faces in a predetermined direction; a plurality ofmicrophones, disposed so as to correspond to the respective speakerunits, and collecting the sound of a noise and outputting an acousticsignal corresponding to the sound of the noise that has been collected;first control signal output means provided to correspond with each ofthe respective speaker units, and that output, on the basis of theacoustic signal that has been output from a first correspondingmicrophone that corresponds to the corresponding speaker unit, a firstcontrol signal for emitting a control sound that reduces the directsound or the diffracted sound of the noise that has been collected bythe first corresponding microphone; second control signal output meansprovided to correspond with each of the respective speaker units, andthat output, on the basis of the acoustic signal that has been outputfrom a second corresponding microphone that corresponds to one of thespeaker units that is adjacent to the corresponding speaker unit, asecond control signal for emitting a control sound that reduces thedirect sound or the diffracted sound of the noise that has beencollected by the second corresponding microphone; third control signaloutput means provided to correspond with each of the respective speakerunits, and that output, on the basis of the acoustic signal that hasbeen output from a third corresponding microphone that corresponds tothe other one of the speaker units that is adjacent to the correspondingspeaker unit, a third control signal for emitting a control sound thatreduces the direct sound or the diffracted sound of the noise that hasbeen collected by the third corresponding microphone; and control means,provided so as to input the signals to the respective speaker units, andcontrolling such that the first control signal is input simultaneouslyto the respective speakers of the speaker units to which the signals arebeing input, such that the respective second control signals are inputto the speakers of the speaker unit to which the signals are being inputwith gradually increasing delay times from the speaker at one end of thespeaker unit to which the signals are being input toward the speaker atthe other end thereof, and such that the third respective controlsignals are input to the speakers of the speaker unit to which thesignals are being input with gradually decreasing delay times from thespeaker at the one end of the speaker unit to which the signals arebeing input toward the speaker at the other end thereof.
 7. The noisereducing device of claim 4, wherein the control means comprises aplurality of accumulators that add together the first signal, one of thesecond control signals and one of the third control signal, and inputsthe result of the addition to the respective speakers of the speakerunit.
 8. The noise reducing device of claim 4, wherein the secondcontrol signal output means comprises: output means, outputting, basedon the acoustic signal that has been output from the second microphone,a control signal that is of opposite phase to the noise that has beencollected by the second microphone; and delay means, delaying aplurality of times the opposite phase control signal respectively bypredetermined incremental periods of time and outputting a plurality ofsecond control signals, and the third control signal output meanscomprises: output means, outputting, based on the acoustic signal thathas been output from the third microphone, a control signal that is ofopposite phase to the noise that has been collected by the thirdmicrophone; and delay means, delaying a plurality of times the oppositephase control signal respectively by predetermined incremental periodsof time and outputting a plurality of third control signals.
 9. Thenoise reducing device of claim 1, wherein the speaker unit(s) is/aredisposed at the upper side of the top edge of a sound barrier, ordisposed at the outside of the upper edge side of a sound barrier, anddisposed so that the predetermined direction faces the propagationdirection of the direct sound or propagation direction of the diffractedsound from a noise.
 10. The noise reducing device of claim 1, whereinthe control signal is corrected according to the external airtemperature.
 11. The noise reducing device of claim 1, furthercomprising detection means, detecting an error signal at a control pointbetween the control sound and direct sound of a noise, or detecting anerror signal at a control point between the control sound and diffractedsound of a noise, and wherein the control signal is corrected so thatthe error signal becomes small by the control signal output means. 12.The noise reducing device of claim 6, wherein the control meanscomprises a plurality of accumulators that add together the firstsignal, one of the second control signals and one of the third controlsignal, and inputs the result of the addition to the respective speakersof the speaker unit.
 13. The noise reducing device of claim 6, whereinthe second control signal output means comprises: output means,outputting, based on the acoustic signal that has been output from thesecond microphone, a control signal that is of opposite phase to thenoise that has been collected by the second microphone; and delay means,delaying a plurality of times the opposite phase control signalrespectively by predetermined incremental periods of time and outputtinga plurality of second control signals, and the third control signaloutput means comprises: output means, outputting, based on the acousticsignal that has been output from the third microphone, a control signalthat is of opposite phase to the noise that has been collected by thethird microphone; and delay means, delaying a plurality of times theopposite phase control signal respectively by predetermined incrementalperiods of time and outputting a plurality of third control signals. 14.The noise reducing device of claim 4, wherein the speaker unit(s) is/aredisposed at the upper side of the top edge of a sound barrier, ordisposed at the outside of the upper edge side of a sound barrier, anddisposed so that the predetermined direction faces the propagationdirection of the direct sound or propagation direction of the diffractedsound from a noise.
 15. The noise reducing device of claim 6, whereinthe speaker unit(s) is/are disposed at the upper side of the top edge ofa sound barrier, or disposed at the outside of the upper edge side of asound barrier, and disposed so that the predetermined direction facesthe propagation direction of the direct sound or propagation directionof the diffracted sound from a noise.
 16. The noise reducing device ofclaim 4, wherein the control signal is corrected according to theexternal air temperature.
 17. The noise reducing device of claim 6,wherein the control signal is corrected according to the external airtemperature.
 18. The noise reducing device of claim 4, furthercomprising detection means, detecting an error signal at a control pointbetween the control sound and direct sound of a noise, or detecting anerror signal at a control point between the control sound and diffractedsound of a noise, and wherein the control signal is corrected so thatthe error signal becomes small by the control signal output means. 19.The noise reducing device of claim 6, further comprising detectionmeans, detecting an error signal at a control point between the controlsound and direct sound of a noise, or detecting an error signal at acontrol point between the control sound and diffracted sound of a noise,and wherein the control signal is corrected so that the error signalbecomes small by the control signal output means.